Use of High Performance Computing in Meteorology, 2005
The first non-beta release of the Weather Research and Forecast (WRF) modeling system in May, 200... more The first non-beta release of the Weather Research and Forecast (WRF) modeling system in May, 2004 represented a key milestone in the effort to design and implement a fullyfunctioning, next-generation modeling system for the atmospheric research and operational NWP user communities. With efficiency, portability, maintainability, and extensibility as bedrock requirements, the WRF software framework has allowed incremental and reasonably rapid development while maintaining overall consistency and adherence to the architecture and its interfaces. The WRF 2.0 release supports the fullrange of functionality envisioned for the model including efficient scalable performance on a range of high-performance computing platforms, multiple dynamic cores and physics options, low-overhead two-way interactive nesting, moving nests, model coupling, and interoperability with other common model infrastructure efforts such as ESMF.
One and two-way nested atmospheric modeling systems are employed in most regional climate simulat... more One and two-way nested atmospheric modeling systems are employed in most regional climate simulations. An alternative refinement approach is to use conforming meshes where resolution varies smoothly. The conforming meshes can alleviate a number of problematic issues inherent in the traditional nested mesh approaches where abrupt non-conforming mesh transitions can lead to grid imprinting and unphysical solutions. We are developing both atmosphere and ocean solvers that use conforming variable-resolution Voronoi meshes for the Model for Prediction Across Scales (MPAS). Preliminary results using the global nonhydrostatic atmospheric MPAS solver on variable resolution meshes support our hypothesis that the smooth mesh transition will significantly improve solutions compared with traditional one- and two-way nested grid approaches. We will show comparisons between seasonal MPAS simulations and those produced with the Weather Research and Forecasting (WRF) model using the Nested Regional...
For the numerical simulation of atmospheric flows that extend as high as the thermosphere, it is ... more For the numerical simulation of atmospheric flows that extend as high as the thermosphere, it is more appropriate to represent the upper boundary of the model domain as a material surface at constant pressure rather than one characterized by a rigid lid. Consequently, in adapting the Model for Prediction Across Scales (MPAS) for geospace applications, a modification of the height-based vertical coordinate is presented that permits the coordinate surfaces at upper levels to transition toward a constant pressure surface at the model’s upper boundary. This modification is conceptually similar to a terrain-following coordinate at low levels, but now modifies the coordinate surfaces at upper levels to conform to a constant pressure surface at the model top. Since this surface is evolving in time, the height of the upper boundary is adaptively adjusted to follow a designated constant pressure upper surface. This is accomplished by applying the hydrostatic equation to estimate the change i...
Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. ... more Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. Since the latter approximation fails at the leading edges of the mutually intruding fluids of lock-exchange flow, solutions to the shallow-water equations can be obtained only through the specification of front conditions. In the present paper, analytic solutions to the shallow-water equations for non-Boussinesq lock-exchange flow are given for front conditions deriving from free-boundary arguments. Analytic solutions are also derived for other proposed front conditions – conditions which appear to the shallow-water system as forced boundary conditions. Both solutions to the shallow-water equations are compared with the numerical solutions of the Navier–Stokes equations and a mixture of successes and failures is recorded. The apparent success of some aspects of the forced solutions of the shallow-water equations, together with the fact that in a real fluid the density interface is a free...
Model has been designed to be an efficient and flexible simulation system for use across a broad ... more Model has been designed to be an efficient and flexible simulation system for use across a broad range of weather-forecast and idealized-research applications. Of particular interest is the use of WRF in nonhydrostatic applications in which moist-convective processes are treated explicitly, thereby avoiding the ambiguities of cumulus parameterization. To evaluate the capabilities of WRF for convection-resolving applications, real-time forecasting experiments have been conducted with 4 km horizontal mesh spacing for both convective systems in the central U.S. and for hurricanes approaching landfall in the southeastern U.S. These forecasts demonstrate a good potential for improving the forecast accuracy of the timing and location of these systems, as well as providing more detailed information on their structure and evolution that is not available in current coarser resolution operational forecast models. 1
The Technical Note series provides an outlet for a variety of NCAR manuscripts that contribute in... more The Technical Note series provides an outlet for a variety of NCAR manuscripts that contribute in specialized ways to the body of scientific knowledge but which are not suitable for journal, monograph, or book publication. Reports in this series are issued by the NCAR Scientific Di-visions; copies may be obtained on request from the Publications Office of NCAR. Designation symbols for the series include: EDD: Engineering, Design, or Development Reports Equipment descriptions, test results, instrumentation, and operating and maintenance manuals. IA: Instructional Aids Instruction manuals, bibliographies, film supplements, and other research or instructional aids. PPR: Program Progress Reports Field program reports, interim and working reports, survey reports, and plans for experiments. PROC: Proceedings Documentation of symposia, colloquia, conferences, workshops, and lectures. (Distribution may be limited to attendees.) STR: Scientific and Technical Reports Data compilations, theore...
The Weather Research and Forecast (WRF) project is a multi-institutional effort to develop an adv... more The Weather Research and Forecast (WRF) project is a multi-institutional effort to develop an advanced mesoscale forecast and data assimilation system that is accurate, efficient, and scalable across a range of scales and over a host of computer platforms. The first release, WRF 1.0, was November 30, 2000, with operational deployment targeted for the 2004-05 time frame. This paper provides an overview of the project and current status of the WRF development effort in the areas of numerics and physics, software and data architecture, and single-source parallelism and performance portability.
The performance of two-way nesting for large eddy simulation (LES) of PBL turbulence is investiga... more The performance of two-way nesting for large eddy simulation (LES) of PBL turbulence is investigated using the Weather Research and Forecasting model framework. A pair of LES-within-LES experiments are performed where a finer-grid LES covering a smaller horizontal domain is nested inside a coarser-grid LES covering a larger horizontal domain. Both LESs are driven under the same environmental conditions, allowed to interact with each other, and expected to behave the same statistically. The first experiment of the free-convective PBL reveals a mean temperature bias between the two LES domains, which generates a nonzero mean vertical velocity in the nest domain while the mean vertical velocity averaged over the outer domain remains zero. The problem occurs when the horizontal extent of the nest domain is too small to capture an adequate sample of energy-containing eddies; this problem can be alleviated using a nest domain that is at least 5 times the PBL depth in both x and y. The sec...
The Weather Research and Forecasting (WRF) Model has been designed to be an efficient and flexibl... more The Weather Research and Forecasting (WRF) Model has been designed to be an efficient and flexible simulation system for use across a broad range of weather-forecast and idealized-research applications. Of particular interest is the use of WRF in nonhydrostatic applications in which moist-convective processes are treated explicitly, thereby avoiding the ambiguities of cumulus parameterization. To evaluate the capabilities of WRF for convection-resolving applications, real-time forecasting experiments have been conducted with 4 km horizontal mesh spacing for both convective systems in the central U.S. and for hurricanes approaching landfall in the southeastern U.S. These forecasts demonstrate a good potential for improving the forecast accuracy of the timing and location of these systems, as well as providing more detailed information on their structure and evolution that is not available in current coarser resolution operational forecast models.
The formulation of a fully compressible nonhydrostatic atmospheric model called the Model for Pre... more The formulation of a fully compressible nonhydrostatic atmospheric model called the Model for Prediction Across Scales–Atmosphere (MPAS-A) is described. The solver is discretized using centroidal Voronoi meshes and a C-grid staggering of the prognostic variables, and it incorporates a split-explicit time-integration technique used in many existing nonhydrostatic meso- and cloud-scale models. MPAS can be applied to the globe, over limited areas of the globe, and on Cartesian planes. The Voronoi meshes are unstructured grids that permit variable horizontal resolution. These meshes allow for applications beyond uniform-resolution NWP and climate prediction, in particular allowing embedded high-resolution regions to be used for regional NWP and regional climate applications. The rationales for aspects of this formulation are discussed, and results from tests for nonhydrostatic flows on Cartesian planes and for large-scale flow on the sphere are presented. The results indicate that the s...
Cut cells use regular or nearly regular polygonal cells to describe fields. For a given orography... more Cut cells use regular or nearly regular polygonal cells to describe fields. For a given orography, some cells may be completely under the mountain, some completely above the mountain, and some are partially filled with air. While there are reports indicating considerably improved simulations with cut cells, inaccuracies may arise with some approximations, producing noise in fields near the surface. This behavior may depend strongly on the approximations made for the advection terms near the surface. This paper investigates the accuracy of advection for numerical schemes for a nondivergent flow near a mountain surface. The schemes use C-grid staggering with densities located at cell centers or on the corners of cells. Also, a nonconserving scheme is considered, which was used in the past with real-data cut-cell simulations. Since the cut cells near the surface create an irregular resolution, the accuracy and order of some approximations may break down near the surface. The objective ...
Idealized simulations on a reduced-radius sphere can provide a useful vehicle for evaluating the ... more Idealized simulations on a reduced-radius sphere can provide a useful vehicle for evaluating the behavior of nonhydrostatic processes in nonhydrostatic global atmospheric dynamical cores provided the simulated cases exhibit good agreement with corresponding flows in a Cartesian geometry, and for which there are known solutions. Idealized test cases on a reduced-radius sphere are presented here that focus on both dry and moist dynamics. The dry dynamics cases are variations of mountain-wave simulations designed for the Dynamical Core Model Intercomparison Project (DCMIP), and permit quantitative comparisons with linear analytic mountain-wave solutions in a Cartesian geometry. To evaluate moist dynamics, an idealized supercell thunderstorm is simulated that has strong correspondence to results obtained on a flat plane, and which can be numerically converged by specifying a constant physical diffusion. A simple Kessler-type routine for cloud microphysics is provided that can be readily implemented in atmospheric simulation models. Results for these test cases are evaluated for simulations with the Model for Prediction across scales (MPAS). They confirm close agreement with corresponding simulations in a Cartestian geometry; the mountain-wave results agree well with analytic mountain-wave solutions, and the simulated supercells are consistent with other idealized supercell simulation studies and exhibit convergent behavior.
Use of High Performance Computing in Meteorology, 2005
The first non-beta release of the Weather Research and Forecast (WRF) modeling system in May, 200... more The first non-beta release of the Weather Research and Forecast (WRF) modeling system in May, 2004 represented a key milestone in the effort to design and implement a fullyfunctioning, next-generation modeling system for the atmospheric research and operational NWP user communities. With efficiency, portability, maintainability, and extensibility as bedrock requirements, the WRF software framework has allowed incremental and reasonably rapid development while maintaining overall consistency and adherence to the architecture and its interfaces. The WRF 2.0 release supports the fullrange of functionality envisioned for the model including efficient scalable performance on a range of high-performance computing platforms, multiple dynamic cores and physics options, low-overhead two-way interactive nesting, moving nests, model coupling, and interoperability with other common model infrastructure efforts such as ESMF.
One and two-way nested atmospheric modeling systems are employed in most regional climate simulat... more One and two-way nested atmospheric modeling systems are employed in most regional climate simulations. An alternative refinement approach is to use conforming meshes where resolution varies smoothly. The conforming meshes can alleviate a number of problematic issues inherent in the traditional nested mesh approaches where abrupt non-conforming mesh transitions can lead to grid imprinting and unphysical solutions. We are developing both atmosphere and ocean solvers that use conforming variable-resolution Voronoi meshes for the Model for Prediction Across Scales (MPAS). Preliminary results using the global nonhydrostatic atmospheric MPAS solver on variable resolution meshes support our hypothesis that the smooth mesh transition will significantly improve solutions compared with traditional one- and two-way nested grid approaches. We will show comparisons between seasonal MPAS simulations and those produced with the Weather Research and Forecasting (WRF) model using the Nested Regional...
For the numerical simulation of atmospheric flows that extend as high as the thermosphere, it is ... more For the numerical simulation of atmospheric flows that extend as high as the thermosphere, it is more appropriate to represent the upper boundary of the model domain as a material surface at constant pressure rather than one characterized by a rigid lid. Consequently, in adapting the Model for Prediction Across Scales (MPAS) for geospace applications, a modification of the height-based vertical coordinate is presented that permits the coordinate surfaces at upper levels to transition toward a constant pressure surface at the model’s upper boundary. This modification is conceptually similar to a terrain-following coordinate at low levels, but now modifies the coordinate surfaces at upper levels to conform to a constant pressure surface at the model top. Since this surface is evolving in time, the height of the upper boundary is adaptively adjusted to follow a designated constant pressure upper surface. This is accomplished by applying the hydrostatic equation to estimate the change i...
Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. ... more Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. Since the latter approximation fails at the leading edges of the mutually intruding fluids of lock-exchange flow, solutions to the shallow-water equations can be obtained only through the specification of front conditions. In the present paper, analytic solutions to the shallow-water equations for non-Boussinesq lock-exchange flow are given for front conditions deriving from free-boundary arguments. Analytic solutions are also derived for other proposed front conditions – conditions which appear to the shallow-water system as forced boundary conditions. Both solutions to the shallow-water equations are compared with the numerical solutions of the Navier–Stokes equations and a mixture of successes and failures is recorded. The apparent success of some aspects of the forced solutions of the shallow-water equations, together with the fact that in a real fluid the density interface is a free...
Model has been designed to be an efficient and flexible simulation system for use across a broad ... more Model has been designed to be an efficient and flexible simulation system for use across a broad range of weather-forecast and idealized-research applications. Of particular interest is the use of WRF in nonhydrostatic applications in which moist-convective processes are treated explicitly, thereby avoiding the ambiguities of cumulus parameterization. To evaluate the capabilities of WRF for convection-resolving applications, real-time forecasting experiments have been conducted with 4 km horizontal mesh spacing for both convective systems in the central U.S. and for hurricanes approaching landfall in the southeastern U.S. These forecasts demonstrate a good potential for improving the forecast accuracy of the timing and location of these systems, as well as providing more detailed information on their structure and evolution that is not available in current coarser resolution operational forecast models. 1
The Technical Note series provides an outlet for a variety of NCAR manuscripts that contribute in... more The Technical Note series provides an outlet for a variety of NCAR manuscripts that contribute in specialized ways to the body of scientific knowledge but which are not suitable for journal, monograph, or book publication. Reports in this series are issued by the NCAR Scientific Di-visions; copies may be obtained on request from the Publications Office of NCAR. Designation symbols for the series include: EDD: Engineering, Design, or Development Reports Equipment descriptions, test results, instrumentation, and operating and maintenance manuals. IA: Instructional Aids Instruction manuals, bibliographies, film supplements, and other research or instructional aids. PPR: Program Progress Reports Field program reports, interim and working reports, survey reports, and plans for experiments. PROC: Proceedings Documentation of symposia, colloquia, conferences, workshops, and lectures. (Distribution may be limited to attendees.) STR: Scientific and Technical Reports Data compilations, theore...
The Weather Research and Forecast (WRF) project is a multi-institutional effort to develop an adv... more The Weather Research and Forecast (WRF) project is a multi-institutional effort to develop an advanced mesoscale forecast and data assimilation system that is accurate, efficient, and scalable across a range of scales and over a host of computer platforms. The first release, WRF 1.0, was November 30, 2000, with operational deployment targeted for the 2004-05 time frame. This paper provides an overview of the project and current status of the WRF development effort in the areas of numerics and physics, software and data architecture, and single-source parallelism and performance portability.
The performance of two-way nesting for large eddy simulation (LES) of PBL turbulence is investiga... more The performance of two-way nesting for large eddy simulation (LES) of PBL turbulence is investigated using the Weather Research and Forecasting model framework. A pair of LES-within-LES experiments are performed where a finer-grid LES covering a smaller horizontal domain is nested inside a coarser-grid LES covering a larger horizontal domain. Both LESs are driven under the same environmental conditions, allowed to interact with each other, and expected to behave the same statistically. The first experiment of the free-convective PBL reveals a mean temperature bias between the two LES domains, which generates a nonzero mean vertical velocity in the nest domain while the mean vertical velocity averaged over the outer domain remains zero. The problem occurs when the horizontal extent of the nest domain is too small to capture an adequate sample of energy-containing eddies; this problem can be alleviated using a nest domain that is at least 5 times the PBL depth in both x and y. The sec...
The Weather Research and Forecasting (WRF) Model has been designed to be an efficient and flexibl... more The Weather Research and Forecasting (WRF) Model has been designed to be an efficient and flexible simulation system for use across a broad range of weather-forecast and idealized-research applications. Of particular interest is the use of WRF in nonhydrostatic applications in which moist-convective processes are treated explicitly, thereby avoiding the ambiguities of cumulus parameterization. To evaluate the capabilities of WRF for convection-resolving applications, real-time forecasting experiments have been conducted with 4 km horizontal mesh spacing for both convective systems in the central U.S. and for hurricanes approaching landfall in the southeastern U.S. These forecasts demonstrate a good potential for improving the forecast accuracy of the timing and location of these systems, as well as providing more detailed information on their structure and evolution that is not available in current coarser resolution operational forecast models.
The formulation of a fully compressible nonhydrostatic atmospheric model called the Model for Pre... more The formulation of a fully compressible nonhydrostatic atmospheric model called the Model for Prediction Across Scales–Atmosphere (MPAS-A) is described. The solver is discretized using centroidal Voronoi meshes and a C-grid staggering of the prognostic variables, and it incorporates a split-explicit time-integration technique used in many existing nonhydrostatic meso- and cloud-scale models. MPAS can be applied to the globe, over limited areas of the globe, and on Cartesian planes. The Voronoi meshes are unstructured grids that permit variable horizontal resolution. These meshes allow for applications beyond uniform-resolution NWP and climate prediction, in particular allowing embedded high-resolution regions to be used for regional NWP and regional climate applications. The rationales for aspects of this formulation are discussed, and results from tests for nonhydrostatic flows on Cartesian planes and for large-scale flow on the sphere are presented. The results indicate that the s...
Cut cells use regular or nearly regular polygonal cells to describe fields. For a given orography... more Cut cells use regular or nearly regular polygonal cells to describe fields. For a given orography, some cells may be completely under the mountain, some completely above the mountain, and some are partially filled with air. While there are reports indicating considerably improved simulations with cut cells, inaccuracies may arise with some approximations, producing noise in fields near the surface. This behavior may depend strongly on the approximations made for the advection terms near the surface. This paper investigates the accuracy of advection for numerical schemes for a nondivergent flow near a mountain surface. The schemes use C-grid staggering with densities located at cell centers or on the corners of cells. Also, a nonconserving scheme is considered, which was used in the past with real-data cut-cell simulations. Since the cut cells near the surface create an irregular resolution, the accuracy and order of some approximations may break down near the surface. The objective ...
Idealized simulations on a reduced-radius sphere can provide a useful vehicle for evaluating the ... more Idealized simulations on a reduced-radius sphere can provide a useful vehicle for evaluating the behavior of nonhydrostatic processes in nonhydrostatic global atmospheric dynamical cores provided the simulated cases exhibit good agreement with corresponding flows in a Cartesian geometry, and for which there are known solutions. Idealized test cases on a reduced-radius sphere are presented here that focus on both dry and moist dynamics. The dry dynamics cases are variations of mountain-wave simulations designed for the Dynamical Core Model Intercomparison Project (DCMIP), and permit quantitative comparisons with linear analytic mountain-wave solutions in a Cartesian geometry. To evaluate moist dynamics, an idealized supercell thunderstorm is simulated that has strong correspondence to results obtained on a flat plane, and which can be numerically converged by specifying a constant physical diffusion. A simple Kessler-type routine for cloud microphysics is provided that can be readily implemented in atmospheric simulation models. Results for these test cases are evaluated for simulations with the Model for Prediction across scales (MPAS). They confirm close agreement with corresponding simulations in a Cartestian geometry; the mountain-wave results agree well with analytic mountain-wave solutions, and the simulated supercells are consistent with other idealized supercell simulation studies and exhibit convergent behavior.
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